A thin carbonaceous film has various excellent characteristics such as high hardness, heat conductivity, electric insulating property, transparency, high refraction index, chemical resistance, low friction property, and low abrasion property. In recent years, coating for improving the performance of various substrates has been demanded in view of its excellent environmental compatibility and biocompatibility. Particularly, for a diamond-like carbon film or diamond film, as a thin carbonaceous film having excellent physical properties, improvement of the coating technique has been expected for enhancing mechanical, optical, and electrical functions of various substrates. However, the diamond-like carbon and diamond involve the following problems due to their characteristics at present and development of a new carbon film has been desired for solving such problems.
In the case of forming a carbon film on a substrate using glass as a substrate, it has been expected for prevention of injury at the surface due to high hardness thereof and application to optical devices having new functions due to high refraction index thereof. For example, a method of forming a microcrystal diamond (hereinafter simply referred to as MCD) film on a glass substrate by a CVD method has already been known (for example, refer to Patent Document 1 and Non-Patent Document 1).
For application as an optical protective film, it has been attempted to utilize the high transmittance of a diamond film. It has been known that the transmittance becomes higher as the grain size of the diamond particles coating the glass surface is smaller and the surface roughness is smaller. However, according to conventional CVD methods, since the MCD formed has a grain size as large as from 0.3 μm to several μm, the obtained MCD film lacks in the surface planarity and sufficient transmittance cannot be obtained. For improving the transmittance, it is necessary to form a planar surface by grinding, and the cost therefor is one of causes for inhibiting the popularization.
Accordingly, it has been attempted for developing a method of forming a planar surface without requiring grinding by making the grain size of the diamond particles smaller. However, the conventional methods involve a problem that amorphous carbon exhibiting black color intrudes simultaneously with deposition of diamond having small grain size, to result in a problem of deteriorating the transmittance though the surface becomes planar.
Further, in the case of utilizing a carbon film coating for a glass protective film, high adhesion is required. For example, Patent Document 1 discloses a method of coating diamond on a glass substrate having a good performance according to a tape test. However, in the application use, for example, in the coating of front glass of an automobile or coating on a spectacle lens, coating retaining higher adhesion as well as high transmittance are necessary.
Moreover, for optical application to the lens such as in spectacles, cameras and cinema projectors, it is important that the coating layer has high refraction index and shows no double refractivity. However, in the case of utilizing the CVD method as a general coating method, it is extremely difficult to synthesize diamond not showing double refractivity in view of thermal strains or residual stresses. Further, the density is often lowered, and the refractive index is usually lowered considerably. Accordingly, it has been a problem that the diamond coating is not suitable for the optical application.
Additionally, in view of high hardness, and low friction or low abrasion characteristic, coating of a thin carbon film to sliding portions of mechanical parts made of iron or stainless steel as the material has been expected. However, since the coating of the diamond-like carbon film or the diamond film to an iron substrate involves a problem that a film is not deposited due to the intrusion of carbon atoms as the constituent element of the film into the iron substrate or a problem of making the substrate brittle, practical use thereof is extremely difficult. On the other hand, an intermediate layer forming method of coating the surface of the iron substrate with a thin film of titanium, chromium or a nitride thereof prior to the coating has been developed. However, in view of the problem of requiring a cost for forming the intermediate layer or the problem that the adhesion of the coating is still low, it has been demanded for the development of a coating instead of diamond-like carbon or diamond.
In addition, in the case of using copper as a substrate, it is desired for coating the diamond-like carbon film or the diamond film to an electronic circuit substrate having a copper surface in view of the high electric insulating property thereof. However, deposition of the film to the surface of copper is extremely difficult. Further, even if it is deposited, adhesion of the film to the copper surface is low, which results in a significant problem that the film tends to be peeled soon. In order to improve this problem, formation of an intermediate layer of titanium or a nitride thereof has been attempted as in the iron substrate described above. However, this also results in a problem for the occurrence of cost and a problem of low adhesion. Further, in a case of conducting, particularly, diamond coating by the CVD method which is a usual method, boron in the atmosphere is easily taken into the film with no intentional doping, which results in a significant problem that the electric insulating property of the diamond coating is lowered.
Further, as a method of coating the carbon film to a plastic substrate, coating of a diamond-like carbon film to a PET bottle has been put to practical use. However, it has been demanded for the coating of plastics with diamond for the use at a higher temperature and in the optical utilization. For example, 90% or more of receptacle lenses have been made of plastics in recent years. In the case where the diamond coated plastic spectacle lens can be prepared, it is possible to prevent injuries to the lens or prepare a high functional lens by utilizing the high refraction index of diamond. However, synthesis for the diamond film requires a high temperature of 600° C. or higher at the lowest and, accordingly, diamond is not suitable at present as a coating material for plastics.    Patent Document 1: JP-A-10-95694    Non-Patent Document 1: Diamond and Related Materials vol. 7, pp 1639-1646 (1998)